Such networks are now being used to an increasing extent, for example, as a replacement for incandescent or fluorescent lamps in lighting equipment for automotive vehicles. Similarly they are being increasingly used for indicator lamps, e.g. for signalling the operation of the brake lights, direction indicators, and safety indicating devices.
The light emitting diode networks often used in practice have semi-conductor crystals arranged on electrical connectors, formed in a predetermined configuration on an insulating support element and arranged to provide the electrical supply for the diodes. Each electrical connector is respectively, and selectively, in contact with the p-type and n-type regions of the semi-conductor light emitting crystals constituting the network. This assembly is designed for a method of mounting the electronic components on large surfaces, that is to say a modular assembly, with regular spacing, adapted for direct soldering of the p-type anode-base of the semi-conductor crystals, using conductive resins which only polymerise at temperatures of the order of 200.degree. C. More particularly, they are adapted for soldering of gold wire for connecting the n-type cathode, which provides the other electrical contact zone of each crystal. This method of assembly is poorly adapted for use with those printed circuits that are designed for the mounting of electronic components where the latter have previously been deposited and welded in a vacuum at relatively low temperatures, or with other networks in which the connections are carried on the insulating supports of plastics materials, which would be liable to be degraded by the temperatures necessary for the soldering of the light emitting crystals.
In addition, in most known applications, the electro-luminescent elements, which generally form a matrix of light emitting diodes, have to be arranged in large numbers on a small surface, and the emission of light beams from each of them must be localised into a zone which is substantially a point. The luminous intensity of the beams is then reinforced subsequently, using an appropriate optical system.
French published patent specification No. FR 2 574 616 A discloses such an optical system, in which the assembly comprising a plurality of light emitting diodes, whether or not carried by a specific support inserted on an insulating substrate, together with a system of optical collimating elements, has to respond to a predetermined directionality of the light beam. The collimating elements should preferably constitute an array of parabolic reflectors, each one being on substantially the same optical axis as the corresponding semi-conductor crystal.
Furthermore, in spite of the low current consumption of the electro-luminescent elements, the fact that they are concentrated on the insulating support matrix on which they are carried leads to thermal problems, which also produce side effects in regard to luminous efficiency. As an example, an increase in the temperature of the assembly due to the power dissipated by 50 diodes (of the order of 4 watts) would lead to a reduction in light output which may be as much as 0.7% per degree C. Such an increase in temperature may also lead, in certain cases, or with certain arrangements, to deterioration of the greater part of the materials, such as plastics materials, which generally constitute the reflector or its possible coating layers, the latter being deposited by a metallisation procedure in a vacuum, followed by application of a protective varnish.
In the published French patent application No. FR 2 565 726 A, it has been proposed that light emitting diodes should be fixed on a first metallic band which constitutes an electrical connector of a first polarity, while a second metallic band, constituting an electrical sector of a second polarity, is connected to each of the light emitting diodes by a connecting wire. However, such an arrangement does not allow for sufficiently good removal of the heat which is produced in operation, and it is also somewhat fragile because of the presence of the connecting wires, which may be ruptured or become disconnected under the influence of heating and vibrations which are inherent in use during an automotive vehicle.